Fuel cells using solid polymer electrolyte (SPE) films are expected to find widespread practical use as power supplies for electric cars and small-size auxiliary power supplies due to a low operating temperature below 100° C. and a high energy density. For such SPE fuel cells, constituent technologies relating to electrolyte films, platinum base catalysts, gas diffusion electrodes, and electrolyte film/electrode assemblies are important. Among others, the electrolyte films and electrolyte film/electrode assemblies are one of the most important technologies relating to the performance of fuel cells.
In SPE fuel cells, an electrolyte film on its opposite sides is combined with a fuel diffusion electrode and an air diffusion electrode so that the electrolyte film and the electrodes form a substantially integral structure. Then the electrolyte film not only acts as an electrolyte for conducting protons, but also plays the role of a diaphragm for preventing a fuel such as hydrogen or methanol from directly mixing with an oxidant such as air or oxygen even under applied pressure. From the electrolyte aspect, the electrolyte film is required to have a high ion (proton) transfer velocity, a high ion exchange capacity, and a high and constant water-retaining ability enough to maintain a low electric resistance. The role of a diaphragm, on the other hand, requires the electrolyte film to have a high dynamic strength, dimensional stability, chemical stability during long-term service, and no extra permeation of hydrogen gas or methanol as the fuel and oxygen gas as the oxidant.
Electrolyte films used in early SPE fuel cells were ion exchange films of hydrocarbon resins obtained through copolymerization of styrene with divinyl benzene. These electrolyte films, however, lacked practical usefulness due to very low durability. Thereafter, perfluorosulfonic acid-modified fluororesin films developed by E.I. duPont and commercially available under the trade mark “Nafion” have been widely used instead.
Conventional fluororesin base electrolyte films as typified by Nafion are improved in chemical durability and stability. However, when they are applied to direct methanol fuel cells (DMFC) using methanol as the fuel, a crossover phenomenon that methanol runs through the electrolyte film occurs, resulting in a reduced output. Another problem associated with conventional fluororesin base electrolyte films as typified by Nafion is an increased cost because their manufacture starts from the synthesis of monomers and requires a number of steps. This becomes a substantial bar against practical applications. The ion conductivity must be kept low in order to hold down the crossover of methanol. At the present, there is a trade-off between them. It remains unsolved to reduce the methanol crossover while maintaining a high ion conductivity.
With respect to the thickness of electrolyte films, as the film becomes thinner, proton conduction becomes easier and hence, fuel cells provide better power generation characteristics. Thin electrolyte films, however, suffer from a problem that they can be ruptured when an electrolyte film and electrodes are pressed together at elevated temperature to enhance the bond therebetween.
Efforts have been made to develop inexpensive electrolyte films that can replace the Nafion and similar films. A number of electrolyte films under study are described in Journal of Power Sources, 114 (2003), pp. 32-53. However, these electrolyte films after their film formation are joined to electrodes by pressing at elevated temperatures, which leaves problems of possible rupture of films and complex steps. The joining under heat and pressure does not always achieve a sufficient adhesion.
To improve the level of productivity and adhesion, JP-A 2003-203646 proposes to apply a solution of an electrolyte film in a solvent onto an electrode, and press bond the assembly with the solvent partially left therein. Since the electrolyte film has not been cured, only low adhesion is achieved.
JP-A 2003-217342 and JP-A 2003-217343 disclose crosslinking of electrolyte films for the purpose of improving durability. Since solid electrolyte films are crosslinked, subsequent press bonding at elevated temperatures is necessary to fabricate an electrolyte film/electrode assembly.
Also, WO 03/033576 discloses a method of controlling the fuel permeability of an electrolyte film by impregnating the electrolyte film with a non-electrolyte monomer, followed by polymerization. The non-electrolyte monomer is cured. However, since the film subject to impregnation is in solid form, subsequent press bonding at elevated temperatures is necessary.